The invention is a process for the use of water granulated silicon in the preparation of alkylhalosilanes. The process comprises contacting an alkyl halide described by formula RX, with a particulate water granulated silicon containing greater than 0.5 to about 5.0 weight percent iron, in the presence of a catalyst composition comprising copper, at a temperature within a range of about 250.degree. C. to 350.degree. C., where R is selected from the group consisting of alkyls comprising one to about four carbon atoms and X is a halogen.
The Direct Process for producing alkylhalosilanes is well-known and has been refined and modified in many ways since Rochow first described preparing alkylhalosilanes by contacting alkylhalides with silicon at elevated temperatures. This process is used for producing virtually all commercial alkylhalosilanes in the world today.
Rochow, U.S. Pat. No. 2,380,995, teaches passing a gaseous stream of methyl chloride into a heated tube where it contacted powdered silicon at about 300.degree. C. Rochow obtained a mixture comprising 52 weight percent methyltrichlorosilane, 14.5 weight percent dimethyldichlorosilane, and lessor amounts of other silanes. The reaction between silicon and the gaseous hydrocarbon halide is facilitated by the presence of a metallic catalyst such as copper. Furthermore, Rochow teaches that the copper may be alloyed with the silicon.
Rochow et al., U.S. Pat. No. 2,380,996, teach an improved method where a solid porous contact mass formed of powdered silicon and powdered copper is reacted with a hydrocarbon halide. This method continues to be a method of choice for the commercial production of dialkyldihalosilanes, since use of the powdered materials provide for better control of the process than when a silicon-copper alloy is used.
The requirements on the silicon in terms of chemical composition and particle size distribution for the direct synthesis of methylchlorosilanes from silicon and methyl chloride have been investigated thoroughly. Chemical grade silicon metal employed in the direct synthesis of methylchlorosilanes typically has an elemental composition of 0.100 to 0.280 Wt. % aluminum, 0 to 0.150 Wt. % calcium, 0.150 to 0.500 Wt. % iron and 0.015 to 0.050 Wt. % titanium. Chemical composition can enhance the reactivity and selectivity of the reaction to produce the alkylhalosilanes. However, small amounts of particular elements are known to adversely affect reactivity and selectivity. The silicon particle size and distribution plays a key role in reactivity and selectivity of the reaction as well. Freeburne et al., U.S. Pat. No. 5,312,948, teaches an improved process for the reaction of an alkyl halide with particulate silicon in a fluidized-bed. The improvement comprises controlling the particle size of the silicon to within a range of one micron to 85 microns.
The silicon structural composition and its influence on the reaction with methyl chloride have been the subject of numerous scientific studies. It was determined that the silicon structure can be influenced by the cooling process used during production of the silicon. For example, cooling the silicon by a process such as atomization was found to increase production rates of the direct synthesis of methylchlorosilanes from silicon and methyl chloride. Feldner et al., U.S. Pat. No. 5,015,751 teach a process for the production of organochlorosilanes using silicon produced by atomizing with an inert gas or a suitable alloy of silicon produced by atomizing with an inert gas. The atomized silicon chemical composition was 0.05-1% by weight Fe; 0.01-1% by weight Al; 0.0001-1% by weight Ca; 0-0.5% by weight Na; 0-0.5% by weight Li; 0-0.5% by weight K; 0-0.5% by weight Mg; 0-0.5% by weight Sr; 0-0.5% by weight Ba; 0-0.5% by weight Be; and the remainder other impurities in small amounts.
Pachaly et al., U.S. Pat. No. 5,334,738, teach the preparation of methylchlorosilanes from silicon and methylchloride in the presence of a copper catalyst and optional promoter substances. The structural parameter QF of the silicon employed in the method is determined by (a) cutting up silicon test specimens to form a cut surface, (b) totaling on the cut surface the areas of precipitates of intermetallic phases having a longitudinal shape to give an area number A, (c) totaling on the cut surface the areas of precipitates of intermetallic phases having a circular shape to give an area number B, and (d) obtaining the quotient of the area number A and the area number B, called the structural parameter QF. Pachaly et al. disclosed employing water granulated silicon in the method.
Margaria, U.S. Pat. No. 5,605,583, teaches using metallurgical silicon containing, by weight, 0.25% iron with controlled microstructure for the preparation of halogenosilanes. The microstructure is characterized by an image obtained with a scanning electron microscope. Degen et al., U.S. Pat. No. 5,380,903, teach metallic silicon for the Rochow-Synthesis is reduced to particles measuring at least 5 mm in their smallest dimension and 15 mm in their largest dimension, cooling the silicon from a temperature of at least 700.degree. C. to at most 120.degree. C. within a maximum of 2 seconds, and then ground and reacted.
The present invention provides a process for using less expensive particulate water granulated silicon containing higher levels of iron and titanium in the Direct Process for the preparation of alkylhalosilanes. The inventors have unexpectedly discovered that less expensive particulate water granulated silicon performance in the Direct Process is comparable to that of the higher cost conventionally cast silicon containing lower levels of iron.